Nuclear magnetic resonance is a phenomenon exhibited by a select group of atomic nuclei and is based upon the existence of nuclear magnetic moments in these nuclei (termed "NMR active" nuclei). When an NMR active nucleus is placed in a strong, uniform and steady magnetic field, the spin magnetization of the nucleus precesses at a natural resonance frequency known as the Larmor frequency, which is characteristic of each nuclear type and is proportional to the applied field strength at the location of the nucleus. Typical NMR active nuclei include .sup.1 H (protons), .sup.13 C, .sup.19 F and .sup.31 P. The resonant frequencies of the nuclei can be observed by monitoring with an RF receiver the transverse magnetization which results after a strong RF pulse is applied at, or near, the Larmor frequency.
In order to use the NMR phenomenon to obtain an image of a sample, a magnetic field is applied to the sample, along with a magnetic field gradient which depends on physical position so that the field strength at different sample locations differs. When a field gradient is introduced, as previously mentioned, since the Larmor frequency for a particular nuclear type is proportional to the applied field strength, the Larmor frequencies of the same nuclear type will vary across the sample and the frequency variance will depend on physical position. By suitably shaping the applied magnetic field and processing the resulting NMR signals for a single nuclear type, a nuclear spin density image of the sample can be measured. Because the measured NMR signal is a function of the total number of nuclei of a given type, it is common to use a nucleus which is found in abundance in the sample to be imaged. For example, .sup.1 H (protons) are commonly used because they are abundant in many materials and therefore, generate a large NMR signal.
FIG. 1A illustrates a portion of a prior art NMR imaging apparatus. As discussed above, the sample must be placed in a uniform magnetic field, a field gradient must be applied and RF pulses must also be applied in order to obtain the image. Accordingly, in conventional NMR spectrometers, the sample is usually mounted in a "probe" device for performing the actual imaging experiment. In FIG. 1A, probe 1 is utilized to hold and lower the sample into a magnet chamber (not shown) which provides the constant magnetic field. The probe comprises a hollow body 2. Passing through body 2 is a hollow copper tube 4. Several insulating platform plates 6, 8 and 10 are transversely mounted on the end of copper tube 4 which support and physically space the probe components. The sample is actually positioned within an RF coil 3 that serves a number of functions as described below. The RF coil is, in turn, surrounded by a gradient coil set 12 mounted on a hollow insulting form 14 which slides over coil 3 and electrically connects to the NMR probe by means of plug 16 and socket 20. In conventional systems, the coil set 12 may, for example, contain a Golay coil which generates a magnetic field gradient in a known manner. The coil set is positioned over coil 3 by means of platform plates 6, 8 and 10 which closely fit to the inner diameter 22 of coil form 14. A dewar 19 generally extends through tube 4 to allow cool or hot air to be blown over coil 3 to provide the system with the capability of varying the temperature of the coil during operation.
In order to properly accomplish NMR imaging, the probe device utilized should satisfy several design parameters. First, it is important to use an RF coil with a size that approximates that of the sample being imaged in order to make the system efficient and to improve the signal-to-noise ratio. More particularly, RF coil 3 serves two purposes. First, it transmits the strong RF pulse to the sample that is required to nutate the spin magnetization into a plane transverse to the static magnetic field direction and, second, it is used to receive the NMR signals generated by the nuclei. With regard to RF pulse transmission, it is well-known that physically larger coils require more energy than smaller coils to generate an RF field of a given strength at points within the coil. Since a coil sized slightly larger than the sample will deliver sufficient RF energy at all points within the sample to perform the experiment, a coil that is much larger than the sample will generate a significant field outside of the sample, thereby wasting much of the energy utilized to generate the RF pulse. Consequently, such a system is inefficient.
With regard to signal reception, only portions of the RF coil that are close to the sample are capable of gathering the weak NMR signals. Those portions of the coil remote from the sample receive only noise. Therefore, the RF coil should be approximately equal in size to the sample being imaged to ensure that it will gather all the NMR signals emitted from the sample without unduly increasing the amount of noise received.
Consequently, in order to facilitate the imaging of various size samples, the prior art system is capable of accommodating various RF inserts 15, each having a differently-sized RF coil so that the prior art probe can easily accommodate different-sized samples. Such an insert is shown in FIG. 1B. Each RF insert 15' includes a disk insulator 18' which incorporates a pair of connectors 13' which slide over the extension posts 11 in order to physically and electrically connect insert 15' to probe 1. Each RF insert 15' also includes a pair of leads 17' that extend from connectors 13' to electrically connect to RF coil 3'.
In the probe, RF coil 3 is connected to adjustable capacitors 5 to form a resonant circuit. Each adjustable capacitor 5 is connected to a tuning handle 7 via a tuning rod 9 so that the capacitors can be manually adjusted, thereby enabling the RF resonance circuit to be tuned.
For reasons known to those skilled in the art, it is important to center the RF coil in the middle of the constant magnetic field and the gradient field. Consequently, when in position, the RF insert is slid over extension posts 11 until the coil is centered. In order to accommodate coils of relatively large diameters, the RF inserts 15' are designed so that the coil centers are positioned several inches away from the insulator disk 18'. When large diameter RF coils are used, the length of leads 17 which connect the coil to the system is relatively short and does not significantly interfere with system performance. However, when smaller RF coils are utilized, in order to physically center coil 3 properly, relatively long leads 17 must be used to connect coil 3 to the connectors 13. At the resonance frequencies used in a typical NMR system, the stray inductance introduced by the leads 17 into the resonant circuit becomes significant in relation to the inductance of the smaller RF coil 3. This stray inductance does not contribute to either RF field generation or NMR signal gathering because it is positioned at a significant distance away from the sample. Therefore, the large stray inductance significantly reduces the efficiency of the probe circuit.
Accordingly, it is an object of the present invention to provide an efficient NMR probe for small samples.
It is another object of the present invention to provide an NMR probe which can accommodate samples of various sizes with efficient imaging capability.
It is still another object of the present invention to provide apparatus which can be used with existing imaging probes to more efficiently image small samples.
It is yet another object of the present invention to provide small sample imaging apparatus which can be used with existing .mu.-imaging probes.
It is a further object of the present invention to provide apparatus which can be used with existing .mu.-imaging probes to more efficiently image small samples without permanently modifying the existing probe.
It is still a further object of the present invention to provide small sample imaging apparatus which replaces the prior art RF coil insert with apparatus that efficiently utilizes a small diameter coil.
It is yet a further object of the present invention to provide small sample imaging apparatus which replaces the prior art RF coil insert and reduces stray capacitance to improve efficiency.